Selective head-to-tail dimerization of phenylacetylene catalyzed by a diruthenium μ-methylene complex

Article abstract of DOI:10.1016/S0020-1693(02)01498-6

The μ-methylene complex [Ru₂(μ-CH₂)(CO) ₄(μ-dppm)₂] (1) in toluene solution at 75°C is the first known catalyst for the selective dimerization of PhC≡CH or of PhC≡CD to give the head-to-tail dimer PhC≡CC(Ph)=CH₂ or PhC≡CC(Ph)=CD₂ respectively, probably by a mechanism involving alkenyl-alkynyl group coupling at the diruthenium center. After many turnovers, a new diruthenium complex was detected and identified as [Ru₂{μ- η²-PhCCH}(CO)₄(μ-dppm)₂] (2) which was not an active catalyst for the dimerization reaction, and the organic product PhCCMe was also detected at this stage. Complex 2 was prepared more readily by reaction of PhCCH with [Ru₂(μ-CO)(CO)₄(μ-dppm) ₂] (3) in toluene at 75°C. The reaction of 1 with a large excess of PhCCH gave further oligomerization and the major products were identified as an octamer and hexamer of PhCCH. Complex 1 was a catalyst for the dimerization of 1-hexyne to a mixture of dimerization products E-BuC≡C-CH=CHBu and BuC≡C-C(Bu)=CH₂ in a ratio of about 2:1, and also for polymerization of HCCH, but it failed to react with the alkynes PhCCMe, PhCCPh or 3-hexyne at 75°C in toluene.

Full text of DOI:10.1016/S0020-1693(02)01498-6

Inorganica Chimica Acta 350 (2003) 101Á106
/
www.elsevier.com/locate/ica
Selective head-to-tail dimerization of phenylacetylene catalyzed by a
diruthenium m-methylene complex
Yuan Gao, Richard J. Puddephatt *
Department of Chemistry, University of Western Ontario, London, Ont., Canada N6A 5B7
Received 9 May 2002; accepted 3 August 2002
Dedicated to Pierre Braunstein in honor of his many distinguished contributions to metal cluster chemistry and catalysis.
Abstract
The m-methylene complex [Ru2(m-CH2)(CO)4(m-dppm)2] (1) in toluene solution at 75 8C is the first known catalyst for the selective
dimerization of PhCÅ
/
CH or of PhCÅ
/
CD to give the head-to-tail dimer PhCÅ
/
CC(Ph)Ä
/
CH2 or PhCÅ
/
CC(Ph)Ä
/
CD2 respectively,
probably by a mechanism involving alkenylÁ
/
alkynyl group coupling at the diruthenium center. After many turnovers, a new
diruthenium complex was detected and identified as [Ru2{m-h2-PhCCH}(CO)4(m-dppm)2] (2) which was not an active catalyst for
the dimerization reaction, and the organic product PhCCMe was also detected at this stage. Complex 2 was prepared more readily
by reaction of PhCCH with [Ru2(m-CO)(CO)4(m-dppm)2] (3) in toluene at 75 8C. The reaction of 1 with a large excess of PhCCH
gave further oligomerization and the major products were identified as an octamer and hexamer of PhCCH. Complex 1 was a
catalyst for the dimerization of 1-hexyne to a mixture of dimerization products E-BuCÅ
/
CÃ
/
CHÄ
/
CHBu and BuCÅ
/
CÃ
/
C(Bu)Ä/CH2 in
a ratio of about 2:1, and also for polymerization of HCCH, but it failed to react with the alkynes PhCCMe, PhCCPh or 3-hexyne at
75 8C in toluene.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Head-to-tail dimerization; Diruthenium complexes; AlkenylÁalkynyl groups
/
1. Introduction
catalysts since they can show high regio- and stereo-
selectivity (Table 1), and the mechanisms have been
studied in detail [4]. The formation of enynes in most
cases was found to occur mostly by head-to-head
The development of efficient catalytic systems for the
formation of new carbonÃ
/
carbon bonds is one of the
most challenging topics in organic synthesis [1]. The
direct coupling of two terminal alkynes to give an enyne
is of high interest because enynes exist in nature and
may serve as building blocks for natural products or for
further structure elaboration [2]. Since the initial dis-
coveries [2,3] that the dimerization of 1-alkynes to
enynes with transition metal compounds as catalysts
dimerization of RCÅ
CHR via the alkynylÁ
described in Scheme 1(a) [2Á
diruthenium complex catalyst is also believed to act by
the alkynylÁvinylidene coupling mechanism although
involving both metal centers throughout as shown in
Scheme 1(b), and it also gives a linear trimer [5]. The E-
or Z-stereochemistry is controlled by the nature of the
/
CH to give E- or Z-RCÅ
vinylidene coupling mechanism
4]. The single known
/
CCHÄ
/
/
/
/
can occur either by alkynylÁ
/
vinylidene coupling
(Scheme 1) [2,3] or by alkynylÁvinyl coupling (Scheme
/
alkynylÁvinylidene coupling and only one of the possi-
ble stereochemistries is shown in each of Scheme 1(a and
b). The less common formation of enynes via head-to-
/
2) [3], there has been considerable activity in the study of
transition-metal-mediated dimerization of terminal al-
kynes [4]. Mononuclear ruthenium complexes are useful
tail dimerization of RCÅ
follows the alkynylÁvinyl coupling mechanism illu-
strated in Scheme 2 [2Á4]. Clearly, it is not possible to
/
CH to give RCÅ
/
CCRÄ/CH2
/
/
* Corresponding author. Tel.: ꢀ
3022.
E-mail address: pudd@uwo.ca (R.J. Puddephatt).
/
1-519-661 2111; fax: ꢀ1-519-661
/
form the head-to-tail dimer by the vinylidene mechan-
ism of Scheme 1, but Scheme 2 can lead to either
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0020-1693(02)01498-6

102
Y. Gao, R.J. Puddephatt / Inorganica Chimica Acta 350 (2003) 101Á106
/
Since the development of binuclear and cluster catalysts
remains a considerable challenge [8], it was decided to
explore the chemistry of alkynes with the new diruthe-
nium methylene complex [Ru2(m-CH2)(CO)4(m-dppm)2]
[9] and the results are reported below.
2. Results and discussion
2.1. Dimerization of PhCÅ
/CH by [Ru2(m-
CH2)(CO)4(m-dppm)2]
At room temperature, the m-methylene complex
[Ru2(m-CH2)(CO)4(m-dppm)2] (1) in toluene solution
was inactive toward excess PhCÅ
warming to 75 8C, the slow formation of 2,4-diphenyl-
but-1-en-3-yne, PhCÅCC(Ph)ÄCH2, was observed ac-
cording to Eq. (1), and identified by its NMR spectrum
and by GCÁMS. Using a molar ratio of PhCÅCH:1 of
8:1, the conversion of PhCÅCH to PhCÅCC(Ph)ÄCH2
/CH. However, on
/
/
/
/
/
/
/
was complete within 24 h, with unchanged complex 1
the only ruthenium complex present in detectable
quantity, as monitored by 1H and 31P NMR. The
product PhCÅ
/
CC(Ph)Ä/CH2 is formed by the less
common head-to-tail dimerization of PhCCH (Schemes
1 and 2), and none of the expected head-to-head
Scheme 1.
dimerization products E- or Z-PhCCCHÄ
/CHPh was
detected [2Á5].
/
ð1Þ
At low concentrations of PhCCH, the rate of the
catalytic reaction in toluene solution at 75 8C was
roughly first order with respect to PhCCH, as shown
by data in Table 2, but at higher concentrations there
was a saturation effect. After many turnovers, a new
diruthenium complex was detected and identified as
[Ru2{m-h2-PhCCH}(CO)4(m-dppm)2] (2). Complex 2
was not an active catalyst for the dimerization reaction,
and so the catalytic activity decreased as 2 was formed
from 1. As complex 2 was formed, the organic product
PhCCMe was also detected and identified both by its 1H
Scheme 2.
RCCCRÄ
/
CH2 or to E-RCCCHÄ
the orientation of the insertion of the alkyne into the
MH bond.
/
CHR, depending on
As well as this general interest in alkyne dimerization,
there has been interest in stoichiometric reactions of
alkynes with both mononuclear and binuclear ruthe-
nium complexes. For example, the coupling of alkynes
at the diruthenium centers in the m-methylene complex
[Ru2(m-CH2)(m-CO)(CO)2(Cp)2] has been reported [6],
while the reactions of alkynes with [Ru2(m-CO)(CO)4(m-
NMR spectrum and by GCÁMS. The slow formation of
/
2, is therefore, described by Eq. (2). Complex 2 was
prepared more readily by reaction of [Ru2(m-
CO)(CO)4(m-dppm)2] (3) with PhCCH in toluene at
75 8C, according to Eq. (3). Complex 3 was a poor
catalyst for the dimerization of PhCCH under these
conditions, and only 5% conversion of PhCCH was
observed under conditions where 1 gave 100% conver-
sion
dppm)2], dppmꢁ
/
Ph2PCH2PPh2, gave alkyne, alkenyl
or vinylidene complexes without alkyne coupling [7].

Y. Gao, R.J. Puddephatt / Inorganica Chimica Acta 350 (2003) 101Á
/106
103
Table 1
Some catalytic systems for the dimerization of 1-alkynes
Catalyst
Monomer Enyne (yield)
T (8C), time (h), conversion
(%)
Reference
[RhCl(PPh3)3]
[RhCl(PPh3)3]
[RuHPhL] a
1-Hexyne BuCCÃ
/
C(Bu)Ä
C(Ph)ÄCH2 (14%), PhCCÃ
CHÄCHPh, E: 95%, Z: 5%
CHÄCHPh, E: 76%, Z: 16%, PhCCÃ
/
CH2
20, 30, 64
20, 30, 34
50, 12, 65
[2a]
[2a]
[3e]
[4k]
PhCCH
PhCCH
PhCCH
PhCCÃ
PhCCÃ
PhCCÃ
(8%)
t-BuCCH t-BuCCÃ
/
/
/
CHÄCHPh (20%)
/
/
/
RuCp*[C(Cy)NHCÄ
CHN(Cy)]Cl
/
/
/
/
C(Ph)ÄCH2 20, 0.05, 100
/
RuH2(CO)(PPh3)3
RuClTp(PPh3)2
RuCp*(PPh3)CCPh
/
CHÄ
CHÄ
CHÄ
Z-FcCCÃCHÄCHFc
/CHBu-t, E: 85%, Z: 15%
100, 10, 51
110, 20, 97
20, 20, 75
60, 30, 62
[3d]
[4d]
[4b]
[5]
PhCCH
PhCCH
FcCCH
PhCCÃ
/
/
CHPh, E: 76%, Z: 16%
CHPh, E: 80%, Z: 20%
PhCCÃ
/
/
b
ꢃ
Ru2Cp2 (SPr)2R
/
/
a
Lꢁ
/
P{(CH2)3PMe2}3.
b
RꢁC(ÄCHFc)CCFc, Fcꢁ
/
/
/
ferrocenyl.
peaks at 1992, 1988, 1980 and 1950 cmꢂ1, and a peak at
1590 cmꢂ1 due to the CÄ
C stretching vibration of the
coordinated PhCÄCH group.
Table 2
Catalytic dimerization of PhCCH in the presence of complex 1 (0.0135
mmol) at 75 8C for 24 h
/
/
The corresponding dimerization of PhCCD at 75 8C,
with complex 1 as catalyst, was monitored by 2H NMR,
while the ruthenium complexes present were identified
PhCÅ
/
CH (mmol)
Monomer conversion (%)
TON
0.11
0.27
0.54
0.90
100
77
8
15
28
29
by 31P NMR. After 24 h, the enyne PhCCÃ
/
C(Ph)Ä/CD2
69
43
was observed as the only organic product, and 1 was the
only ruthenium complex. Over longer reaction periods,
the organic product PhCCCH2D was detected as com-
plex 1 was slowly converted to 2. Formation of
PhCCCH2D clearly involves combination of PhCCD
and CH2, derived from the m-methylene group of 1.
2.2. The mechanism of the dimerization reaction
ð2Þ
The observations described above do not define the
mechanism of reaction but, in combination with stoi-
chiometric reactions studied previously [7], it is possible
to make a reasoned hypothesis. The reaction of complex
3 with PhCCH in more polar solvents acetone or
dichloromethane does not occur as shown in Eq. (3),
but in the stepwise manner shown in Scheme 3. The first
product is the hydrido(m-alkynyl) complex 4, which then
ð3Þ
Complex 2 was characterized by its spectroscopic
data, by comparison with the spectra of similar com-
plexes [Ru2{m-h2-RCCH}(CO)4(m-dppm)2], for exam-
ple, with the analogous complex having Rꢁ
The 31P NMR spectrum of complex 2 contained two
multiplets centered at dꢁ32.5 and 34.5 for the phos-
phorus atoms of the dppm ligands. The 1H NMR
spectrum exhibited a singlet resonance at dꢁ8.20 due
to the PhCÄCH proton and two multiplets at dꢁ4.05
and 2.60 for the CH2P2 protons of the dppm ligands. In
13CO labeled sample of 2, four terminal carbonyl
/CO2Me [7].
/
/
/
/
a
resonances were observed at dꢁ218, 209, 208 and 207
/
in the 13C NMR spectrum. Consistent with the proposed
structure, the IR spectrum showed four terminal CO
Scheme 3.

104
Y. Gao, R.J. Puddephatt / Inorganica Chimica Acta 350 (2003) 101Á106
/
reacts with more PhCCH to give the alkenyl(m-alkynyl)
complex 5 and finally the alkenyl(m-alkynyl)(m-alkyli-
dene) complex 6. Coupling of the alkenyl and alkynyl
block formation of the inactive m-alkyne complex 2.
However, the electronic effects could also be important
in labilizing carbonyl ligands to generate vacant sites for
reactivity and in activating the alkyne and releasing the
product enyne.
groups in 5 or 6 would give the dimer PhCCC(Ph)Ä
/
CH2,
and this is probably a key step in the mechanism of
dimerization when the catalyst is 3.
2.3. Oligomerization of phenylacetylene
By analogy with Scheme 3, the reaction of PhCCH
with complex 1 may occur to give the hydrido(alk-
ynyl)(methylene) complex 7, and then the alkenyl(m-
alkynyl)(m-methylene) complex 8 (Scheme 4). The pro-
The reaction of complex 1 with a large excess of
PhCCH (1:67) at 75 8C in toluene-d8 was monitored by
1H and 31P NMR spectroscopy. The catalysis to give
duct PhCCC(Ph)Ä
/
CH2 is then formed by reductive
PhCCC(Ph)Ä/CH2 occurred as described previously, and
elimination with coupling of the alkenyl and alkynyl
groups, and addition of PhCCH regenerates 7 as shown
in Scheme 4. Complex 1 can then be considered as a
catalyst precursor to the active complexes 7 and 8 that
were not detectable. This proposed mechanism is a
complex 1 was slowly converted to 2 over 2 days.
However, further slow reactions occurred over a period
of 10 days, with conversion of complex 2 to an
uncharacterized mixture of ruthenium products and
with disappearance of essentially all PhCCH and
binuclear variant of the alkenylÁalkynyl coupling
/
PhCCC(Ph)Ä
/
CH2. By precipitation from this solution,
mechanism of Scheme 2. The mechanism of Scheme 1,
that has been observed with most other ruthenium-
a yellow powder was isolated whose 1H NMR spectrum
was similar to the reported spectrum of atactic poly(phe-
nylacetylene) [10]. However, GPC analysis indicated
that this yellow powder contained a mixture of oligo-
meric products with an octamer and a hexamer as the
main components. In agreement, the MS analysis of the
based catalysts [2Á5], including the only previous case of
/
a binuclear catalyst [5], is not capable of giving the
observed head-to-tail dimer and so can be eliminated.
The termination is likely to occur according to
Scheme 5. If the reaction of PhCCH with 1 occurs in
such a way that the hydride forms cis to the m-methylene
group, as in 9, then these groups are likely to combine to
form a methyl group in complex 10. Reductive coupling
of the methyl and alkynyl groups then forms the organic
product PhCCMe, and addition of PhCCH across the
sample showed major peaks at m/zꢁ
612 (hexamer), with less intense peaks at m/zꢁ
/
816 (octamer) and
714
/
(heptamer) and 510 (pentamer). Given that most
PhCCH was consumed before these products were
formed, it is likely that, for example, the octamer is
really a tetramer of the dimer PhCCC(Ph)Ä
mechanism of this subsequent oligomerization is ob-
scure.
/
CH2, but the
RuÃ/Ru bond occurs to give the inactive complex 2.
Again, neither proposed intermediate 9 or 10 was
observed and so the detailed structures are speculative.
However, the overall Scheme 5 is fully consistent with
the formation of PhCCCD2H from the m-CD2 complex
1-d2. It is also noted that reactive methyldiruthenium
complexes are known to be formed by reaction of 1 with
other protic reagents [9].
2.4. The reaction of complex 1 with other alkynes
Complex 1, as a solution in toluene at 75 8C, reacted
slowly with excess 1-hexyne to give a mixture of
dimerization products E-RCÅ
/
CÃ
/
CHÄ
/
CHR and RCÅ
/
Overall, the role of the m-methylene group in enhan-
cing the catalytic activity is probably in part just to
CÃ
/
C(R)Ä
/
CH2 (Rꢁbutyl) in a ratio of about 2:1. These
/
products, formed by head-to-head and head-to-tail
dimerization respectively, were identified by their known
NMR spectra [11] and confirmed by GCÁMS. A slow
/
transformation of complex 1 to unidentified ruthenium
complexes was also observed during this process. This
catalytic reaction is both slower and less selective than
the corresponding reaction with PhCCH described
above.
The reaction of complex 1 in CD2Cl2 at 20 8C led to
slow precipitation of a fine purple solid and the
corresponding reaction in toluene-d8 at 75 8C gave a
similar product more rapidly. This material was largely
insoluble in common organic solvents, and was tenta-
tively identified as polyacetylene. Complex 1 remained
essentially unreacted during this process. Complex 1
failed to react with the alkynes PhCCMe, PhCCPh or 3-
hexyne at 75 8C in toluene.
Scheme 4.

Y. Gao, R.J. Puddephatt / Inorganica Chimica Acta 350 (2003) 101Á
/106
105
Scheme 5.
3. Conclusions
4.1.2. Specific example
To a solution of 1 in toluene-d8 (15 mg, 0.0135 mmol)
in a NMR tube was added PhCCH (12 ml, 0.108 mmol),
The solution was kept at 75 8C and monitored with time
by NMR. After 24 h, no signal resonances for PhCCH
could be detected and 2,4-diphenylbut-1-en-3-yne was
found to be the only organic product by1H NMR and
The diruthenium m-methylenc complex [Ru2(m-
CH2)(CO)4(m-dppm)2] (1) displayed unusual catalytic
activity toward the dimerization and oligomerization of
the terminal alkyne PhCCH. In toluene solution at
75 8C, complex 1 is a catalyst for the selective dimeriza-
tion of PhCÅ
/
CH to give PhCCC(Ph)Ä
/
CH2 and appears
GCÁ
NMR data in toluene-d8: 2,4-diphenylbut-1-en-3-yne
[11]: d(1H)ꢁ
5.7 [d, Jꢁ1 Hz, HC(H)ÄC]; 5.9 [d, Jꢁ
Hz, HC(H)ÃCÃ]; GCÁMS: m/zꢁ
204 (Mꢀ). n-BuCCÃ
C(n-Bu)CÄ 5.3 [d, Jꢁ6 Hz, ÃCÄ
CH2 [11]: d(1H)ꢁ
CHAHB]; 5.1 [d, Jꢁ6 Hz, ÃCÄCHAHB]; GCÁMS: m/
zꢁ
164 (Mꢀ), E-n-BuCCCHÄCH-n-Bu [11]: d(1H)ꢁ
6.04 [dt, Jꢁ16, 6 Hz, BuCHÄ CHÃ]; 5.1 [d, Jꢁ16 Hz,
BuCHÄCH Ã]; GCÁMS: m/zꢁ
164 (Mꢀ).
/
MS.
to be the first complex to display such selectivity.
/
/
/
/
1
/
/
/
/
/
/
/
/
/
/
/
/
/
/
4. Experimental
/
/
/
/
/
/
/
All manipulations were carried out under a dry
nitrogen atmosphere using either standard Schlenk
techniques or a glove box. Toluene was dried and
distilled immediately before use. [Ru2(m-CH2)(CO)4(m-
dppm)2] was synthesized by the reaction of [Ru2(m-
/
/
/
/
4.2. 2H NMR studies of the dimerization of PhCÄ
/
CD
CO)(CO)4(m-dppm)2] with CH2N2 [9]. H, 13C, and 31P
1
To a solution of 1 (15 mg, 0.0135 mmol) in toluene in
an NMR tube was added PhCCD (20 ml, 0. 178 mmol).
The solution was kept at 75 8C for 24 h and then
checked by 2H NMR for the deuterium distribution.
NMR spectra were recorded using a Varian Inova 600
or 400 spectrometer. Mass spectra were recorded using a
Finnigan Mat 8200 spectrometer. IR spectra were
recorded by using a PerkinÁElmer PE2000 FT IR.
/
NMR in toluene-d8: d(2H)ꢁ
C(Ph)ÄCD2]; 1.6 [s, PhCCH2D]. GCÁ
206(Mꢀ).
/
5.6 and 5.7 [s, PhCCÃ
/
/
/
MS: m/zꢁ
/
4.1. The dimerization of alkynes in the presence of
complex 1
5. [Ru2{m-h1:h1-C(Ph)C(H)}(CO)4(m-dppm)2] (2)
4.1.1. General procedure
To a solution of 1 (15 mg, 0.0135 mmol) in toluene-d8
(0.5 ml) in an NMR tube was added a known amount of
the alkyne. The course of the reaction was then
monitored at room temperature or at 75 8C by NMR.
The extent conversion of the alkyne to the dimeric
product(s) was determined by integration of the respec-
a) To a solution of 1 in toluene-d8 (15 mg, 0.0135
mmol) in a NMR tube was added PhCCH (30 ml,
0.270 mmol). The solution was kept at 75 8C for 48
h and at this stage, only complex 2 and a small
amount of 1 were present as the ruthenium com-
pounds in solution. NMR of 2 in toluene-d8:
1
tive H resonances.
d(1H)ꢁ
/
8.2 [s, 1H, C(Ph)CH]; 4.0 [m, 2H, PÃ
/

106
Y. Gao, R.J. Puddephatt / Inorganica Chimica Acta 350 (2003) 101Á106
/
CH Ã
/
P]; 2.6 [m, 2H, PÃ
/
CHÃ
/
P]. d(31P)ꢁ
32.5 [m,
/
(d) Y. Wakatsuki, H. Yamazaki, N. Kumegawa, T. Satoh, J.Y.
Satoh, J. Am. Chem. Soc. 113 (1991) 9604;
dppm]; 34.5 [m, dppm].
(e) W.T. Boese, A.S. Goldman, Organometallics 10 (1991) 782.
[4] (a) C.S. Yi, N.H. Liu, Organometallics 17 (1998) 3158;
(b) C.S. Yi, N.H. Liu, A.L. Rheingold, L.M. Liable-Sands,
Organometallics 16 (1997) 3910;
b) To a solution of [Ru2(m-CO)(CO)4(m-dppm)2] (3) in
toluene-d8 (20 mg, 0.018 mmol) in an NMR tube
was added PhCCH (10 ml, 0.09 mmol). The solution
was then kept at 75 8C for 1 h. At this point, the
only complex present in solution was 2. IR (Nujol,
cmꢂ1): 1992, 1988, 1980 and 1950 [terminal CO];
(c) B.M. Trost, M.T. Sorum, C. Chan, A.E. Harms, G. Ruhter, J.
Am. Chem. Soc. 119 (1997) 698;
(d) C. Slugovc, K. Mereiter, E. Zobetz, R. Schmid, K. Kirchner,
Organometallics 15 (1996) 5275;
1590 [Ã
above. For a sample prepared by the reaction of
PhCCH with 13CO labeled 3: d(13C)ꢁ
218 [m,
terminal CO]; 209 [m, terminal CO]; 208 [m,
terminal CO]; 207 [m, tenninal CO]. The product
was obtained as a yellow solid by precipitation with
ether. Anal. Calc. for C62H50O4P4Ru2: C, 62.8; H,
4.2. Found: C, 62.3; H, 4.0%.
/
CÄ
/
CÃ
/
]. The NMR data were as reported
(e) T. Straub, A. Haskel, M.S. Eisen, J. Am. Chem. Soc. 117
(1995) 6364;
(f) P. Barbaro, C. Bianchini, M. Peruzzini, A. Polo, F. Zanobini,
Inorg. Chim. Acta 220 (1994) 5;
/
(g) C. Bianchini, P. Frediani, D. Masi, M. Peruzzini, F. Zanobini,
Organometallics 13 (1994) 4616;
(h) Y. Wakatsuki, N. Koga, H. Yamazaki, K. Morokuma, J. Am.
Chem. Soc. 116 (1994) 8105;
(i) C. Bianchini, M. Peruzzini, F. Zanobini, P. Frediani, A.
Albinati, J. Am. Chem. Soc. 113 (1991) 5453;
(j) M. Schafer, N. Mahr, J. Wolf, H. Werner, Angew. Chem., Int.
Ed. Engl. 32 (1993) 1315;
Acknowledgements
(k) W. Baratta, W.A. Herrmann, P. Rigo, J. Schwarz, J.
Organomet. Chem. 593Á594 (2000) 489.
/
We thank the NSERC (Canada) for financial support.
R.J.P. thanks the Government of Canada for a Canada
Research Chair.
[5] H. Matsuzaka, Y. Takagi, Y. Ishii, M. Nishio, M. Hidai,
Organometallics 14 (1995) 2153.
[6] (a) A.F. Dyke, S.A.R. Knox, P.J. Naish, G.E. Taylor, Chem.
Commun. (1980) 803;
(b) M. Akita, R. Hua, S. Nakanishi, M. Tanaka, Y. Moro-oka,
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